1. Field of the Invention
This invention relates to a chemical vapor deposition method for forming films or coatings of metal oxide films showing a giant magnetoresistive effect.
2. Description of the Related Art
When a conductor carrying current is placed in a magnetic field, its resistance changes. This change is called its magnetoresistance and is denoted .DELTA.R/R.sub.H =(R.sub.O -R.sub.H)/R.sub.H, and may be either plus or minus. The magnetoresistive effect has been used in a number of sensing and measuring devices. Typically these devices use a magnetic material, often a film, as the magnetoresistive element since magnetic materials can have large internal fields for small applied fields. Examples of magnetoresistive devices are magnetic bubble domain sensors, and position and angle sensors in automobiles.
A giant magnetoresistance (GMR) effect was first observed in Fe/Cr multilayers, and subsequently in many other metallic multilayers (e.g. Co/Cu, Fe/Cu), and sandwiches (e.g., Co/Au/Co). At room temperature, mamagnetoresistance changes as high as 40% have been observed in these multilayers, compared to only 2-3% for normal ferromagnetic materials such as permoalloys. A similar GMR effect was also observed in heterogeneous metastable alloys such as Cu--Co, Ag--Co and Fe--Cu. Recently, the magnitude of the GMR effect that is available has been increased dramatically by the use of certain oxide materials. A thousand fold magnetoresistance change has been reported in magnetic manganese oxide films grown epitaxially on LaAlO.sub.3 substrates by laser ablation.
Potential commercial applications of films of GMR materials are in magnetic sensors,magnetoresistive read heads, and magnetoresistive random access memories, where they offer significant advantages over conventional magnetic materials. However, several problems in the earlier known metallic multilayer and metastable alloy GMR materials persist. The most serious one is the higher magnetic drive fields required to observe the effect, typically in the several hundred to several thousand Oersteds (Oe); the required sensitivity range for many applications lies in the range of 10 to 100 Oe. Other limitations include low sheet resistivity and poor thermal stability. The new oxide GMR materials, which are based on manganese oxides, may overcome these problems.
The new manganese oxide-based GMR materials have formula: EQU La.sub.x A.sub.1-x MnO.sub.3
wherein A is selected from the group consisting of barium, calcium, and strontium, and x is a number in the range of from 0.2 to 0.4. The magnetic manganese oxide (La.sub.1-x A.sub.x)MnO.sub.3, where A represents Ca, Sr, or Ba, has a perovskite-type crystal structure with ferromagnetic ordering in the a-b planes and antiferromagetic ordering along the c-axis below the Neel temperature. The ferromagnetically ordered Mn-O layers of the a-b planes are separated by a nonmagnetic La(A)-O monolayers. This spin structure is intrinsic, and is similar to that of the metallic multilayers described above which exhibited GMR.
(La.sub.1-x A.sub.x)MnO.sub.3 compounds having the extreme values x=0,1 are neither ferromagnetic nor good electrical conductors; they are semiconductors. Only compounds with intermediate values of x are ferromagnetic, with the strongest ferromagnetism occurring in the range 0.2&lt;x&lt;0.4. Within this same range, the electrical conductivity is high. At 100 K the conductivity decreases by a factor of 0.01 as x is decreased to 0.1 or below or increased to 0.6 or above. This result has been explained on the basis of the mixed Mn.sup.+3 -Mn.sup.+4 valence state in this compound. In the extreme composition LaMnO.sub.3, each metal atom is triply charged. However, if one replaces some of the La atoms by divalent atoms such as Ca, Sr, or Ba, a corresponding number of Mn atoms becomes quadruply charged.
Because of the GMR effect depends strongly on composition, the ability to use the oxide GMR films in sensors and other devices where consistency is important will depend on the ability to control the stoichiometry of the deposited films. In addition to the dependence on the mole ratio of lanthanum to Group II metal, the GMR properties depend on the amount of oxygen present in the films. Therefore, a successful deposition method will require good control of all aspects of stoichiometry. In addition, a desirable deposition method should be capable of being scaled up, should integrate into current electronic device manufacturing technology, and should not require temperatures so high that they are incompatible with other materials present in the devices.
La.sub.x A.sub.1-x MnO.sub.3 films have been deposited by physical vapor deposition methods such as sputtering and laser ablation. (La.sub.x Ba.sub.1-x)MnO.sub.z thin films prepared by laser ablation showed .DELTA.R/R.sub.H =(R.sub.O -R.sub.H)R.sub.H as large as -150% at room temperature. Sputtered (La.sub.0.72 Ca.sub.0.25)MnO.sub.3 ((LaCa)MnO3) films on MgO substrates gave .DELTA.R/R.sub.H values of -110% at 220 K but 0% at room temperature. The .DELTA.R/R.sub.H of laser deposited (La.sub.0.72 Ca.sub.0.25)MnO.sub.z films on LaAlO.sub.3 substrates was improved to -500% at 100 K but again was O% at room temperature. Annealing these samples in oxygen enhanced their .DELTA.R/R.sub.H value to -100,000% near 77 K, -1500% at 190 K and -400% at 280 K. It has also been reported that films deposited at higher oxygen partial pressures (300 mTorr) have a higher .DELTA.R/R.sub.H near room temperature.
Chemical vapor deposition (CVD) is a particularly attractive method for forming these layers because it is readily scaled up to production runs and because the electronics industry has a wide experience and an established equipment base in the use of CVD technology which can be applied to new CVD processes. In general, the control of key variables such as oxygen partial pressure during deposition, film stoichiometry and film thickness, and the coating of a wide variety of substrate geometries is most feasible with CVD. Forming the thin films by CVD will permit the integration of these materials into existing device production technologies. CVD also permits the formation of layers of the refractory materials that are epitaxially related to substrates having close crystal structures.
The use of CVD to form films showing the GMR effect has not previously been reported. The advantages of CVD in regard to scale-up and manufacturability of the La.sub.x A.sub.1-x MnO.sub.3 GMR films are so attractive that development of a CVD method is an important goal in the commercialization of devices using the GMR effect. Such CVD films would desirably have high GMR effects, comparable to or greater than effects observed in prior art films.
CVD requires that the element source reagents must be sufficiently volatile to permit gas phase transport into the deposition reactor. The element source reagent must decompose in the reactor to deposit only the desired element or its oxide at the desired growth temperatures. Premature gas phase reactions leading to particulate formation must not occur, nor should the source reagent decompose in the lines before reaching the reactor deposition chamber. When compounds are to be deposited, obtaining optimal properties requires close control of stoichiometry which can be achieved if the reagents can be delivered into the reactor in a controllable fashion. In addition, the reagents must not be so chemically stable that they do not react in the deposition chamber.
Thus a desirable CVD reagent is fairly reactive and volatile. Many potentially highly useful refractory materials have in common that one or more of their components are elements, such as the Group II metals barium, calcium, or strontium, or early transition metals zirconium or hafnium, for which few volatile compounds well-suited for CVD are known. In many cases, the source reagents are solids whose sublimation temperature may be very close to the decomposition temperature, in which case the reagent may begin to decompose in the lines before reaching the reactor, and it will be very difficult to control the stoichiometry of the deposited films.
In other cases, the CVD reagents are liquids, but their delivery into the CVD reactor in the vapor phase has proven problematic because of problems of premature decomposition or stoichiometry control.
The problem of controlled delivery of CVD reagents into deposition reactors was addressed by the inventors in U.S. Pat. No. 5,204,314 "Method for Delivering an Involatile Reagent in Vapor Form to a CVD Reactor," and further elaborated in U.S. patent application Ser. No. 08/280,143, "Apparatus and Method for Delivery of Involatile Reagents," which hereby are incorporated herein by reference. As described and claimed in these patents, the delivery of reagents into the deposition chamber in vapor form is accomplished by providing the reagent in a liquid form, as a neat liquid or solution of a solid or a liquid compound, and flowing the reagent liquid onto a flash vaporization matrix structure which is heated to a temperature sufficient to flash vaporize the reagent source liquid. A carrier gas may optionally be flowed by the flash vaporization matrix structure to form a carrier gas mixture containing the flash vaporized reagent.
Upon heating the ensemble of reagents to vaporize and deliver them to the reactor or under the high temperature conditions present in the CVD reactor, the various metal complexes may undergo ligand exchange reactions. If the products of such ligand exchange reactions are involatile, the result may be premature nucleation and formation of particulate species in the reactor, in the lines leading from the source reservoir(s) to the reactor, or in the source reservoir itself, if more than one reagent is held in the same reservoir. Thus stability to ligand exchange reactions in the solvent/reagent system of choice is an important consideration. For CVD of multicomponent oxide materials, compatibility of the reagents requires resistance to ligand exchange reactions.
Therefore, the present invention seeks to provide source reagents, source reagent combinations, solution compositions and deposition conditions that when used with the liquid delivery method enable the reproducible chemical vapor deposition of La.sub.x A.sub.1-x MnO.sub.3 GMR films.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.